Often, during manufacture of semiconductor devices, one or more dies containing the integrated circuit may be bonded to a die pad (or paddle) of a leadframe. The process of bonding the die to the leadframe is usually referred to as a die attach process. The die attach process may be accomplished using an electrically conductive material, such as an adhesive or solder, which connects the die to the leadframe, both mechanically and electrically. The thickness of this conductive material is often referred to as the bond line thickness (BLT).
In the die attach process, the conductive material must allow the bonding to occur between the die and the leadframe while minimizing the formation of voids in the bond. Also, the die attach process must also provide a consistent bond strength across the surface of the die, thereby minimizing localized stresses that may cause fracture or other failure of the semiconductor device. Any voids and inconsistent bond strength in the bond increases the stress and strain on the die, which may lead to cracks and failures in the semiconductor device. In addition, voids may result in inefficient or ineffectual electrical or heat conductivity, potentially causing failures in the semiconductor device. The conductive material, therefore, should have a viscosity sufficiently low to allow for effective bonding by avoiding both of these problems.
FIGS. 1 and 2 illustrate an exemplary die 110 that is attached to die pad 120 by conductive material 130 to form die attach package 100. As shown in FIG. 1, t1 is the BLT between the die 110 and the die pad 120. Increasing the BLT by increasing the thickness of the conductive material 130 decreases the shear stress on the die, which makes larger thicknesses more desirable. However, the low viscosity required to assure no voids usually limits the thickness to less than 3 mils. But increasing the amounts of conductive material used during die attach in effort to increase BLT can result in flow of the conductive material to other portions of the leadframe or the die, potentially causing moisture path, short circuits and problems in wire bonding and often resulting in failure of the semiconductor device.
To avoid these problems, some die attach processes use a “spanker” to flatten the conductive material during the die attach process. However, using the spanker involves extra steps that make the device fabrication process longer, less productive, and more expensive. Additionally, if too much conductive material is used to attempt to achieve a high BLT, the conductive material may be displaced by the spanker from the die pad to other portions of the leadframe, potentially causing short-circuits and other problems.